We plan to examine the hypothesis that the host cell supplies amino acids to the intracellular Toxoplasma gondii. Preliminary experiments with cultured cells have shown that starvation for any one of the amino acids markedly reduces the growth T. gondii. The effect of the amino acid deficiency on host protein synthesis is not significant since we have shown that host protein synthesis is not required for growth T. gondii. These preliminary experiments will be repeated and amplified by attempts to reverse the inhibition by subsequent addition of the absent amino acid. We shall also make a rigorous test of the ability of T. gondii to synthesize several of the non-essential amino acids through the use of mutant host cells. First we shall see if starvation of the host cell for these "non-essential" amino acids has any effect on the growth of T. gondii. If it does we shall incubate the infected cells in medium containing a minimal amount of the "non-essential" amino acid for one day. The infected culture will then be shifted to a medium lacking the amino acid of interest and containing uniformly labeled 14C-glucose. After a short period of growth, the parasites will be purified and their protein extracted and hydrolyzed to amino acids. The amino acid of interest will be separated from others using standard amino acid analyzer techniques and its specific activity determined.